Question

Why is a direct reaction between ${\mathrm{SO}}_3$ and water avoided in the contact process?

Answer 1

Contact process: The contact process is used for the industrial production of sulphuric acid.

• No, we cannot do a direct reaction between ${\mathrm{SO}}_3$(Sulphur trioxide) and ${\mathrm{H}}_2\mathrm{O}$(Water) in the contact process.
• This is because the direct dissolution of Sulphur trioxide in water can cause the high exothermic nature of the reaction. And then acidic vapours are also formed.
• Sulphur trioxide firstly reacts with sulphuric acid and further reacts with water to produce sulphuric acid.

$$\begin{gathered} {\mathrm{SO}}_3\left(\mathrm{g}\right)+{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{l}\right)\to {\mathrm{H}}_2{\mathrm{S}}_2{\mathrm{O}}_7\left(\mathrm{l}\right) \\ \mathrm{conc}.\mathrm{oleum} \\ {\mathrm{H}}_2{\mathrm{S}}_2{\mathrm{O}}_7\left(\mathrm{l}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to \underset{\mathrm{Sulphuric}\mathrm{acid}}{2{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)} \end{gathered}$$

Hence, we can't do a direct reaction between ${\mathbf{SO}}_{\mathbf{3}}$ and ${\mathbf{H}}_{\mathbf{2}}\mathbf{O}$.